Distance-of-flight mass spectrometry (DOFMS) may be explained in comparison to the more conventional technique of time-of-flight mass spectrometry (TOFMS). TOFMS is a known analytical technique commonly employed in a wide range of fields, such as chemistry, biology, medicine, environmental studies, and physics, as well as in a wide range of associated applications. TOFMS measures the time required for ions of varying mass-to-charge ratios (m/z, where m=mass, z=charge) to traverse a known distance. While a number of TOFMS geometries exist, a generic scheme describes the general approach. First, subject ions of various m/z are extracted from an extraction region at substantially the same instant by application of a single, constant electrostatic field. Because all subject ions starting from the same depth in the extraction region are exposed to the same electrostatic field, they gain the same kinetic energy (KE) and, thus, achieve differing velocities (v) dependent on their mass (according to KE=(½)mv2). The subject ions are then allowed to traverse a field-free flight region, wherein the ions separate according to their m/z-dependent velocities. Finally, a mass spectrum is acquired by capturing the time-dependent readout of a suitable detector placed at the exit of the field-free region. Ions of smaller m/z achieve relatively higher velocities and thus reach the detector first, while ions of larger m/z achieve relatively lower velocities and thus require a longer time to reach the detector. The time required by ion to traverse the field-free region is quadratically related to the m/z of the ion.
In contrast to the TOFMS strategy (which measures the time an ion requires to traverse a specified distance), DOFMS separates ions of various m/z ratios according to the distance each ion is able to travel during a specified time period. After velocity separation, the spatial distribution of the subject ions is measured with a detector that possesses spatial resolution. Various embodiments of instruments employing DOFMS are described in U.S. Pat. Nos. 7,041,968 and 7,429,728 and U.S. Patent Publication No. 2008/0017792, the entire disclosures of which are each expressly incorporated by reference herein. Further background considerations regarding DOFMS are described in C. G. Enke et al., “Achievement of Energy Focus for Distance-of-Flight Mass Spectrometry with Constant Momentum Acceleration and an Ion Mirror,” 79 Analytical Chemistry 8650-8661 (2007), the entire disclosure of which is also expressly incorporated by reference herein.
As described in the foregoing references, the subject ions in DOFMS may be accelerated to a constant momentum (rather than a constant energy, as in typical TOFMS) prior to m/z separation in a field-free region, to better focus the ions in space at a specific time. Constant momentum acceleration (CMA) may be achieved by focusing ions of various m/z ratios into a region in which a linear electrostatic field of limited duration is applied (i.e., an “extraction pulse”). The duration of the extraction pulse is purposely restricted so that none of the ions of interest are able to exit the region before the pulse ends. This strategy imparts the same momentum (the product of mass and velocity) to each m/z ratio. Thus, the ion velocities will be inversely related to their m/z ratios. Following CMA, ions separate within the field-free region according to their m/z-dependent velocities. At a specific time, delayed from the application of the extraction pulse, ions will be distributed in space according to 1/(m/z). At that time, a second extraction field is employed to deflect all the ions at an angle, onto the surface of a position-sensitive detector, where the spatial distribution reflects the m/z composition of the subject ions.